Eager for solitude away from the TARDIS and the endlessly inquisitive Leela, the Doctor steps out onto a benign-looking planet. But the apparent tranquillity hides a terrifying secret...

The TARDIS has arrived on a world of violence, where hideous creatures hunt and kill endlessly, vying for supremacy at the top of the food chain. But is evolution on the planet natural or engineered by some higher power? And why has an aggressively suspicious alien police force sent a secret mission here?

With no one safe from the planet's tireless predators, Leela's warrior instincts are tested to the full. The Doctor, meanwhile, begins to suspect that there is a scientific purpose to the planet - one married to a sinister intelligence.

Whatever the data being collated from the planet, the Doctor soon realises its usage may have far-reaching consequences for all humanity...

The Doctor is irritated when Leela attacks the buzzing TARDIS console with her knife, convinced there is a hostile insect inside. When the TARDIS materializes in a pine forest, he sets off for a quiet walk by himself, leaving her locked inside; but moments after leaving the safety of the ship he is targeted and pursued by a predator, something like a giant bird louse. He is forced to climb a tree to escape, but the predator begins to climb the tree as well... Meanwhile, Leela experiments with the TARDIS console and works out how to open the doors and follow the Doctor.

A police squad from the Out-systems Investigation Group has landed nearby in pursuit of an outlaw weapons technologist, a runner from justice. But as the squad advances into the jungle they stop receiving signals from their ship, and Rinandor and Pertandor must return to find out what's happening. They find that the ship has vanished into thin air -- and before they can report back they are attacked by a squad snake, a group organism which attacks with crippling sound waves. Meanwhile, the rest of the group finds the crash site of the runner's ship, but no sign of the ship itself; they are then attacked by flying reptiles which instantly kill Investigator Monly. The others escape, but the group's leader, Kley, now knows that the group was not told everything they need to know about this planet. She'd been told that the group had been fully briefed before she joined them, yet nobody else has been told that the runner is a "toody", one of the second-class citizens of the system from the Second Planet.

Rinandor and Pertandor stumble through an invisible boundary between the jungle and the pine forest, and thus meet Leela. Leela uses her hunting skills to kill the squad snake, saving their lives, but Rinandor remains suspicious of her and demands to know why she's on an interdicted world. Leela easily slips away from them and soon finds the Doctor, who is still trapped by the bird-louse predator. Leela also kills this predator and takes the Doctor back to Rinandor and Pertandor, who are forced to concede that they can't survive on this world without Leela's help. They assume that the Doctor is also a toody -- probably a duelling agent who's brought Leela to this world to train illegally. Trying to find the rest of their group, they instead find a pool blocking their path -- and, impossibly, the equipment packs they discarded during their flight from the snake are on the island in the centre of the pool. Pertandor foolishly tries to swim across to fetch the packs, and is attacked by an aquatic predator. Once again Leela fights and kills the creature, which the Doctor suspects was engineered by a guiding intelligence -- as has this entire situation.

They are reunited with the rest of the OIG squad, and the Doctor learns that they're hunting a weapons technologist and that their expedition is woefully underequipped and underinformed. Leela hears a noise nearby and slips away to investigate, and watches from hiding as three humanoid warriors emerge from a shaft in the ground nearby. The Doctor finds them but is ignored, and theorizes that they're here to provide a challenge for Leela. Unfortunately, the OIG team arrives and tries to confront the three warriors, who instantly react when weapons are drawn. In the ensuing fight two of the warriors are gunned down, the officer Sozerdor is killed, and Leela kills the third warrior in hand-to-hand combat. The shaft seals itself up, and the Doctor theorizes that they're in a weapons development facility -- and that more than just budgetary reasons are behind the OIG team's lack of preparation. He suggests waiting to see what happens next, but then the ground opens up beneath their feet...

The Doctor awakens in darkness, taunted by the voice of the runner. The Doctor refuses to discuss his arrival on the planet, and is then rescued from his force-field prison by Kley's second-in-command, Fermindor, who claims that he awoke alone in this underground complex. They follow pulses of hypnotic light to a recycling centre, where the remains of the two ships from the surface have been reduced to their component molecules -- and where Fermindor's attention is instantly drawn to the undamaged TARDIS, which is circling the plasma stream. His fellow investigator Belay also arrives, having freed himself somehow, and like Fermindor he instantly asks the Doctor about the TARDIS -- confirming the Doctor's suspicion that they have been 'programmed' by the runner to seek more information on the TARDIS. Overpowered by their programmed instincts, they both leap into the plasma stream before he can stop them, and are blown apart.

The runner, meanwhile, sends a false distress call back to the OIG team's homeworld, where Director Drew makes the unprecedented decision to send a three-ship rescue team despite the drain this will cause on the OIG's budget. This act is largely dismissed as a publicity stunt by the media, whose attention is fully occupied by a forthcoming death duel on the public sports channel; the fight between a firster and toody will carry the extra baggage of symbolising the underlying resentment and prejudice between the First and Second Planets. Drew's deputy Feerlenator suspects that there's more going on than he knows -- but even he doesn't realize that Drew is involved in a conspiracy to overthrow the domination of the First Planet.

Leela is rescued from her force-field prison by a clone of the Doctor who tries to elicit information about the TARDIS from her. The real Doctor finds them, and the clone attacks him; but as it is an imperfect copy it is killed by the hypnotic pulses of light. The Doctor and Leela follow the corridor to the facility's control centre, where Leela finds she is able to use the system to generate predators and artificial environments for battle. The Doctor realizes that mental feedback from the machine is amplifying and honing her aggressive tendencies. They are then confronted by the runner -- Monly, who claims that he slipped away from the group when they first landed and replaced himself with a clone who was killed to divert suspicion.

Monly generates four clones of Leela, who march the Doctor and the real Leela to the real control centre. All this time, they -- and the rest of the OIG team -- have been trapped in force-field cubicles and fed sensory illusions. Monly continues to taunt the Doctor, but the Doctor quickly realizes that "Monly" is really a clone. A second Monly emerges from another cubicle, but the Doctor isn't fooled twice. The real runner is finally forced to show himself -- and it's Sozerdor, who has come to this world to manufacture weapons for the conspiracy back home.

Sozerdor traps Leela in a force field and threatens to crush her unless the Doctor reveals the secrets of the TARDIS, but the Doctor manages to convince the confused Monly clones that Sozerdor is insane. They attack Monly, who guns them down -- but the distraction enables the Doctor to rescue Leela. Sozerdor flees while the Doctor and Leela release the others from their cubicles; Kley, Fermindor, Rinandor and Pertandor seem none the worse for their experiences, but Belay seems to be on the verge of losing control. The Doctor explains that they're in a testing facility abandoned
by the extinct Lentic Empire; soldiers were placed in these arenas and tested to destruction, the idea being to breed the perfect super-soldier from the last survivor. Now Sozerdor intends to generate an army of Leela clones -- the ultimate warrior, distilled to perfection by the Last-Man-Running scenario. Sozerdor's distress call and Drew's machinations have resulted in three ships arriving in orbit around the planet -- to provide their clone army with transport back home.

The Doctor and Leela transport themselves and the OIG team to the surface, where clones of the OIG team have been sent out to lure the rescue party into a false sense of security. Leela tries to scout ahead to find out what's happening, but Belay follows and is seen, provoking a gun battle between the clones and the rescue team. Belay, trying to defend himself, loses his sense of his own identity when he sees himself and his friends shooting at him, and accidentally guns down the rescue party before being shot by Rinandor's clone. The other OIG teams manage to defeat their own clones, and on the Doctor's advice, they take the rescue shuttles back to the orbiting ships and advise the commander of the rescue mission to bomb the planet from orbit.

Leela fights her clones as they emerge one by one from the transmat system, and although the strain on her sanity is nearly too much for her to handle the Doctor is there to see her through. They return to the Lentic facility to confront Sozerdor, and are able to trap him in a force field; Leela then activates all of the controls she can find, drawing on the facility's power until the defenses are too weak to shield against the orbital strafing. The power drain also releases Sozerdor from his force field, but before he can shoot the Doctor or Leela, the recycling facility shuts down -- thus cutting off the plasma stream around the TARDIS -- and the lights go out. Sozerdor is disoriented in the darkness, but Leela unerringly locates the TARDIS -- and she and the Doctor escape while the OIG rescue mission bombs the facility, bringing it down around Sozerdor's head.